The pancreatic juice is a clear, strongly alkaline albuminous fluid, containing ferments. These are, however, only brought to their full activity by mixture with the succus entericus and bile, and this is especially the case with the proteolytic ferment. Pancreatic juice has very little action on protein, but is converted into a powerful agent by the addition of a small quantity of intestinal juice. This is due to a ferment in the succus entericus, known as enterokinase, which has the property of activating the trypsinogen of the pancreatic juice to form trypsin. Trypsin has, in an alkaline medium, a strong proteolytic action, forming first alkali-albumin, then albumoses, peptone, and aminoacids. Pancreatic juice also contains an amylase, called amylopsin, and the fat splitting ferment steapsin. The alkalinity of the juice is proportional to the acidity of the chyme which it neutralizes. Pawlow has brought forward evidence that the amount of each ferment in the pancreatic juice varies with the nature of the food, but these results have not yet been fully confirmed by other workers.
The albumoses and peptones of the chyme are broken down by trypsin, to a large number of bodies belonging to the class of aminoacids. The action is a far reaching one and is a process of hydrolysis, although the products show some differences from those of hydrolysis by acid. It is found experimentally that the digestion of protein by pancreatic juice is most complete when, as is the case in the body, the protein has been first subjected to the action of the gastric juice. The following (from Aders Plimmer) are some of the chief substances formed :-
These bodies are the fractions of which protein is made. We shall see later that the proteins of the body have a characteristic structure which is by no means the same as that of those in the food. In the process of digestion the complex molecules of the food proteins having been broken down into simpler ones, these are picked out in certain proportions to build up the tissues of the body, in the same way as the stones which have served for one house may be used again to construct a new building of a different architecture.
The amylolytic ferment of the pancreatic juice has a similar action upon starches to that of ptyalin, the chief product being maltose with a little dextrose. Its activity is doubled, according to Pawlow, by the presence of bile. In young animals a lactase is also present which converts lactose to dextrose and galactose.
The lipase, or fat splitting ferment, converts fats into fatty acids and glycerine : bile increases its action threefold.
Both pancreatic and gastric juice contain ferments which throw caseinogen out of solution, the rennin ferments. It is probable that their action is not identical in the two cases. These ferments are closely connected with pepsin and with trypsin. They do not exist for the sole purpose of clotting milk, for they are present in animals which take none, for instance, in fishes. Some observers have put forward the view that they play a part in the synthesis of protein, but this is by no means proved.
Glaessner, in a case of pancreatic fistula, has investigated the properties of the juice in man and was able to confirm the facts which had been established by experiment upon animals. Five to eight hundred cubic centimetres of juice were secreted daily by his patient.
The bile contains water, mucin, the bile salts - sodium taurocholate and glycocholate - the bile pigments, cholesterin, lecithin, and inorganic salts. The assistance given by the bile in the digestion of fat is due to the tauro- and glycocholate of soda, which have the property of lessening the surface tension between the fats and the intestinal fluids and by this means aiding emulsification and enabling the lipase to come into closer relationship with the fat. The bile salts, by dissolving the fatty acids or soaps produced, also aid their absorption. The function of the cholesterin is not understood. It has been regarded as an excretory product of nervous tissue, but J. A. Gardner has recently shown that if there be no cholesterin in the food there i3 none in the faeces, although it is still contained in the bile; it has long been held that the bile salts are absorbed from the intestine and re-secreted in the bile to help again in the digestion of fat, and it appears probable from these researches that cholesterin is also re-absorbed. Lecithin has been found by Parker and Moore to aid the solution of soaps by bile salts. The pigments of the bile are true excretory products derived from the haemoglobin of the blood.
The Succus entericus is an alkaline fluid containing small quantities of protein, mucin, leucocytes and epithelial cells, and at least three ferments. We have already referred to one of these, enterokinase. Another ferment called erepsin (Cohn-heim) breaks down albumoses and peptones to the various aminoacids enumerated above, and its presence in the small intestine ensures that any protein not fully disintegrated by pepsin and by trypsin shall not escape. Erepsin is also able to attack fibrin and caseinogen. An invertin ferment or invertase is also present which inverts cane sugar to dextrose and levulose; a maltase which converts maltose into dextrose; and, in young animals, a lactase. The carbo-hydrates are by these means entirely hydrolyzed to dextrose, levulose or galactose. It is not, however, certain that this takes place entirely in the lumen of the gut, for it is probable that the ferments, to some extent at least, produce their effect within the mucous membrane.
The digestion and absorption of the food is materially aided by the movements of the small intestine. It has been shown by direct observation and by the X-rays that during digestion the gut is continually kept in motion by the passage along its muscular walls of circular constrictions which have the effect of dividing up the contents into small lengths without moving them onwards. These short sausage shaped pieces are then further divided into two, each of which joins with a similar piece lying above or below it. The movements are repeated again and again with the result that the food materials and the digestive juices are thoroughly mixed, and every part of the whole is brought into intimate relation with the absorbing wall of the gut. At intervals a different kind of movement is seen, the peristaltic wave, which, by a prolonged constriction of a part of the intestine with a relaxation of the region lying immediately below it, propels the contents along the tube.